Reset relief valve

ABSTRACT

A reset relief valve with a body having an inlet port, an outlet port, and an interior wall partially defining an enclosed chamber, a piston disposed in the body and moveable between a first position at which the piston is disposed in fluid flow-blocking relationship between the inlet port and the outlet port of the body and a second position at which the piston is removed from that position, a piston having a head with opposed end surfaces one of which defines a portion of the enclosed chamber and at least one fluid flow passageway extending through the head and providing fluid communication between the enclosed chamber in the body, and a second chamber above a moveable disc in the piston. A preferred embodiment includes the crank assembly with cross bar support and a T-bar to prevent downward movement of the disc.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of patent applicationSer. No. 10/773,972 filed Feb. 6, 2004 which is based on provisionalapplication Ser. No. 60/445,690, filed on Feb. 7, 2003 and provisionalapplication Ser. No. 60/467,640 filed May 3, 2003.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

DESCRIPTION OF ATTACHED APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

This invention relates generally to the field of pressure relief valvesand more specifically to a reset relief valve.

Pressure relief valves are important components in certain fluid flowsystems in which undesirable high pressure may occur. In such systems,overpressure can damage valuable processing equipment or cause breakdownor rupture of weak components of the system. It is desirable that when apre-determined or pre-designated threshold pressure is exceeded, thepressure relief valve open very quickly, i.e., virtuallyinstantaneously, to relieve system pressure. Pressure relief valveshaving this capability are commonly referred to as “pop-off” valvesbecause of their rapid opening action. In carrying out the rapid openingof the valve, movable port-blocking elements within the valve, such as aplug or piston, are subjected to high acceleration and velocity,resulting in severe impact forces being imposed on the piston and otherelements of the valve when the moving member reached its limit of travelaway from the port-blocking position.

In prior art, the working fluid has been used to lessen impact within avalve by routing the fluid through ports or apertures in a manner toreduce impact forces. For example, U.S. Pat. No. 5,715,861 to Williamsshows a fluid flow path whereby the subject fluid flows above the pistonto equalize pressure. Fluid hydraulics using the pumped or working fluidhave not proven to be effective in that high impact forces often tearinternal parts upon initial activation of the valve. In other solutions,cushion members formed of rubber and similar resilient elastomericmaterials have been used to attenuate the impact of the flow controlpiston subsequent to its rapid displacement from the flow-blockingposition. Examples of such valves and rubber cushion members aredisclosed in U.S. Pat. Nos. 2,973,776 and 2,973,777 respectively issuedon Mar. 7, 1961 to Herbert Allen and John N. Troxell, Jr. While theannular elastomeric cushion rings do provide effective initial shockattenuation, the rings, aided by the initial drop in pressure at theinlet port when the piston is moved from its blocking position, promoteundesirable rebounding of the piston, which inhibits the free flow offluid to the outlet port, and impose additional repetitive shock forceson the entire valve assembly. Furthermore, the rebounding actionattributed to the resilient rings often resulted in the relief valveresetting itself before release of the excess pressure, only to betriggered soon thereafter by the unrelieved pressure. This opening andunintended resetting cycle could continue for a significant period oftime until it was detected, and was a major contributor to wear andfailure of the internal linkage mechanism and other components of thevalve.

The various solutions in the prior art fail to provide a relief valvethat can be manually reset after activation and that withstands thepressures commonly found in their application thereby avoiding damagingimpacts on the valve and the need to replace the valve after it isactivated.

BRIEF SUMMARY OF THE INVENTION

The primary advantage of the invention is to provide a pressure reliefvalve that is re-settable after being activated.

Another advantage of the invention is to provide a relief valve thatutilizes hydraulic pressure to dampen activation.

Another advantage of the invention is to provide a relief valve thatprovides a rapid flow path upon activation.

A further advantage of the invention is to provide a relief valve thathas a strengthened crank assembly to withstand high pressures.

Yet another advantage of the invention is to provide an improved pistonfor withstanding impact forces upon activation.

Other advantages of the present invention will become apparent from thefollowing descriptions, taken in connection with the accompanyingdrawings, wherein, by way of illustration and example, an embodiment ofthe present invention is disclosed.

In accordance with a preferred embodiment of the invention, there isdisclosed a reset relief valve having a body with an inlet port, anoutlet port, and an interior wall partially defining an enclosedchamber, a piston disposed in the body and moveable between a firstposition at which the piston is disposed in fluid flow-blockingrelationship between the inlet port and the outlet port of the body anda second position at which the piston is removed from fluidflow-blocking position, the piston having a head which partially definesa first chamber and at least one flow passageway extending between thechamber in the body and a second chamber having a lower surface definedin part by a disc disposed in a hollow portion of the piston head, abonnet attached to the body, a stem bushing disposed in the bonnet inspaced relationship with the head of the piston, the stem bushing havinga surface defining another portion of the first chamber and an internalbore in which the stem is slidably supported, at least one spring and atleast two pivotally interconnected link members operatively disposedbetween the spring and the stem, and a crank assembly for selectivelyengaging the pivotally interconnected link members for urging the pistoninto the first position.

In accordance with a preferred embodiment of the invention, there isdisclosed a reset relief valve with a body having an inlet port, anoutlet port, and an interior wall partially defining an enclosedchamber, a partially hollowed out piston disposed in said body andmoveable between a first position at which the piston is disposed influid flow-blocking relationship between the inlet port and the outletport of the body and a second position at which the piston is removedfrom the fluid flow-blocking position, piston having a head with opposedsurfaces one of which defines a first chamber and at least one fluidflow passageway extending through the head and providing fluidcommunication between the first chamber in said body and a secondchamber having a lower surface defined by an inner piston disposed in ahollow portion of the piston head, a bonnet attached to the body, a stembushing disposed in the bonnet in spaced relationship with the head ofthe piston, the stem bushing having a surface defining another portionof the enclosed chamber and an internal bore in which the stem isslidably supported, crank assembly for urging the piston into the firstposition.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and includeexemplary embodiments to the invention, which may be embodied in variousforms. It is to be understood that in some instances various aspects ofthe invention may be shown exaggerated or enlarged to facilitate anunderstanding of the invention.

FIG. 1 is a partially exploded cross sectional view of a preferredembodiment of the invention.

FIG. 2 is a plan view of a preferred embodiment of the invention withthe valve set in the unactivated position.

FIG. 3 is a plan view of a preferred embodiment of the invention withthe valve set in the open or activated position.

FIG. 4 is an elevational view of a preferred embodiment of the inventionFIG. 5A is a cross sectional plan view of the piston used in a preferredembodiment of the invention.

FIG. 5B is a cross sectional elevation view of the piston used in apreferred embodiment of the invention.

FIG. 6A is a top plan view of the crank used in a preferred embodimentof the invention.

FIG. 6B is a bottom plan view of the crank used in a preferredembodiment of the invention.

FIG. 6C is a side elevational view of the crank used in a preferredembodiment of the invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed descriptions of the preferred embodiment are provided herein.It is to be understood, however, that the present invention may beembodied in various forms. Therefore, specific details disclosed hereinare not to be interpreted as limiting, but rather as a basis for theclaims and as a representative basis for teaching one skilled in the artto employ the present invention in virtually any appropriately detailedsystem, structure or manner.

Turning now to FIG. 1, there is shown a partially exploded crosssectional view of the reset valve of the present invention. In thepreferred embodiment of the present invention, shown in FIGS. 1-4, apressure relief valve 10 has a body 12, a piston 14 movably disposedwithin the body 12, and a bonnet assembly 16. FIGS. 1-4 are shown in ahorizontal orientation relative to the typical assembled vertical fieldposition. References herein to an upward or downward movement or upperand lower positions refer to an orientation where bonnet assembly 16 ispositioned above piston 14 in a vertical orientation. The relief valve10 is typically connected to a line conduit 18 in fluid communicationwith a fluid system containing a fluid under pressure, and a dischargeconduit 20 in fluid communication with a fluid containment or storagestructure, sump, drain line, or other arrangement for receiving fluidrelieved from the fluid system.

The valve body 12 has an inlet port 22 that is adapted to receive theline conduit 18, and an internally disposed annular chamber 24 in fluidcommunication with an outlet port 26 that is adapted to receive thedischarge conduit 20. The body 12 also has an internal bore providing aninterior cylindrical wall surface that partially defines enclosedchambers 28 and 33, the features and function of which are disclosedbelow in more detail.

Piston 14 has a partially hollow head 30 and an elongated stem 32extending outwardly from head 30. A stem bushing 34 is mounted in thebonnet assembly 16 such that the bottom surface of the bushing 34 isspaced from the piston head 14 when the piston 14 is at a first, or set,position as shown in FIGS. 1 and 2.

Disposed in a cylindrical cavity in said piston head 30 is a cylindricalinner piston 39 having a piston cap 40. Inner piston 39 is a round discthat freely moves in sealed engagement within said cavity of saidpiston. Inner piston 39 has at its upper portion piston cap 40 attachedwith cap screw 43. Inner piston 39 is freely moveable within said cavityin piston head 30 and the upper surface of piston cap 40 defines thelower portion of lower enclosed chamber 28. At the first position, thehead 30 and inner piston 39 block the flow of fluid between the inletport 22 and the outlet port 26. The head has an annular upper surface 36that defines, in part, a lower wall of upper enclosed chamber 33. Whenpiston head 14 is at its first, or set position, the lower portion ofinner piston 39 is in fluid contact with the inlet conduit. Piston head30 also has an exhaust port 42 which is shown as a rectangular cut-outportion of the piston head.

Importantly, the piston 14 has one or more apertures 41 (shown in FIGS.2 and 5A) that extend through the upper portion of head 30 of the piston14 providing a hydraulic fluid flow path between lower enclosed chamber28 and upper enclosed chamber 33. The key function of the apertures 41is to provide a controlled flow path for hydraulic fluid to pass betweenupper and lower chambers 28 and 33 when the piston 14 is at its firstand second positions. Chambers 28 and 33 may be filled with any varietyof fluids, including, among others, oil, anti-freeze, water, orlubrication grease. In addition, in certain applications, said chambersmay be filled with other materials including gases or air. Said chambersare sealed off from the working fluid by the arrangement of the pistonhead, disc and piston chamber. This controlled hydraulic fluid pathbetween chambers 28 and 33 cushion the compressive force of the piston14 when it is moved from its above-described first position to a second,or relief, position at which the piston head 30 is moved from its flowblocking position between the inlet and outlet ports, 22 and 26. As canbe seen in FIGS. 1 and 3, when piston 14 is raised to the secondposition, there is essentially no impediment to fluid flow between theline conduit 18, which typically contains pressurized fluid, and thedischarge conduit 20 which is typically open to atmosphere, or ambientpressure through exhaust port 42.

In an illustrative example, the pressure relief valve 10 is installed ina system for pumping drilling mud composed of water, clay and chemicaladditives, down through the inside of a drill pipe of an oil welldrilling operation. The drilling mud is pumped at very high pressure,for example up to 7,500 psi (51,675 kPa), so that the mud is forced outthrough a bit at the lower end of the drill pipe and returned to thesurface, carrying rock cuttings from the well. In this illustrativeexample, the lower surface 38 of the inner piston head 39 has a surfacearea substantially equal to the inner diameter of piston head 30 andpiston head 30 has an outer diameter that is substantially equal to thatof the cross-sectional area of the inlet port 22. The piston stem 32 hasa diameter of about 1.625 in (4.13 cm), thus reducing the surface areaof the upper surface 36 of the piston head 30 to an area less than thearea of surface 38.

In normal operation, with the line, or system pressure at a pressureless than the predetermined value at which it is desired to relievesystem pressure, the piston 14 is at its first position. At thisposition, fluid will flow from the inlet port 22, and stop at the bottomsurface of inner piston 39, until there is essentially equal pressure oninner piston 39 and in turn within enclosed chambers 28 and 33. Exhaustport 42 is positioned so that it does not permit working fluid flow whenthe valve is in its first position to enter annular chamber 24 .However, when the line pressure exceeds the predetermined value,typically as an almost instantaneous pressure spike resulting from lineblockage, the valve 10 will trip in the manner described below, rapidlyraising the piston head 30 away from its first position. Initially, thepiston 14 will raise very rapidly, thereby decreasing the volume of theenclosed chamber 33, until the pressure in the enclosed chamber 33 isgreater than the pressure of the fluid at surface 38 of the inner piston39. As the head 30 continues to move upwardly toward the stem bushing34, the volume of the enclosed chamber 33 decreases and, since thehydraulic fluid oil of this example is essentially a noncompressiblefluid, the rate of upward movement of the piston 14 is restricted by therate at which hydraulic fluid can be moved from the enclosed chamber 33,through the apertures 41 into enclosed chamber 28. Inner piston 39 movesupward slightly as the piston moves into its second position. Thus, theimpact of the piston head 30 against the stem bushing 34, at the upwardlimit of travel of the head 30 is cushioned, and no rebound forces areimposed on the piston head 30 that would cause it move toward the closedposition. Depending upon the viscosity of the hydraulic fluid placed inchambers 28 and 33, both the number and diameter of the apertures mayvary and can be readily determined by one of ordinary skill in the artof fluid mechanics. In the above-described illustrative application inwhich oil is the hydraulic fluid, two apertures 41, each having adiameter of about 0.1875 in (0.476 cm), are equidistantly radiallyspaced from each other around the longitudinal centerline of the pistonhead 30, one at an angle relative to ground of 10 degrees and the otherat an angle relative to ground of 25 degrees. Alternatively, theapertures 41 could be formed by drilling the desired diameter orificethrough replaceable plugs that could be selectively inserted into thepiston head 30. Such an arrangement would provide appropriately sizedapertures for varying fluid viscosity applications, even allowing someof the plugs to have a solid construction if fewer apertures wererequired.

The pressure relief valve 10 embodying the present invention also has afirst means for urging the piston into the above-described firstposition illustrated in FIGS. 1 and 2. With reference to FIG. 1, thefirst means includes a pair of load springs 48 that are controllablycompressed between upper and lower spring retainers 50 and 52. The upperretainer 52 is adjustably mounted on the bonnet assembly 16 by anadjustable nut 54 threadably secured to an adjusting stud 56. Withreference to FIG. 2, the lower end of a load screw 58 is secured to thelower retainer 50 and has an enlarged head at its upper end that fitsinto a mating socket provided at one end of a pivot crank member 55 and,as a result of the downward force applied by the compression springs 48on the lower spring retainer 52, urges the socket end of the pivot crank55 in a downward direction. The pivot crank member 55 is pivotallymounted on a crank pivot pin 62. The second end of the crank member 55biasedly engages the outer ends of two spiral springs 64 that areconcentrically mounted on a top crank pin 66 that is rotatably mountedin the second end of the pivot crank 55. The inner ends of the twospiral springs 64 are secured to the top crank pin 66, such that a smallbias force acting in a counterclockwise direction as viewed in FIG. 2,is applied to the top crank pin 66. The upper end of an upper link 68 isattached to the top crank pin 66 and accordingly moves in concert withthe pin 66 about its axis. The lower end of the upper link 68 ispivotally interconnected with the upper end of a lower link 70 by anupper link pin 72. The lower end of the lower link 70 is pivotallyattached to an upper end of the piston stem 32 by a lower link pin 74.Lower link 70 is also positioned on the other side of the pin so thatalong its entire length is a uniform width which can be pressed againstthe crank. Lower link 70 is flattened and its planar surface has theshape of an oval with two opposing parallel sides and two opposingsubstantially rounded ends. This construction provides added strength tothe linkage and permits proper engagement with the crank as more fullydescribed below.

FIG. 2 shows a plan view of the invention with the reset valve set inthe closed or inactivated position. In this position the fluid flow isprevented due to the presence of the piston and inner piston being in aposition that blocks the flow of fluid from the input port to the outputport.

When the valve 10 is in its operatively set position, with the pistonhead 30 at the first, or flow-blocking position, the upper and lowerlinks 68 and 70 are vertically aligned thereby forming an essentiallyrigid line such that the compression force of the springs 48 istransferred through the load screw 58 to the pivot crank 55, thencethrough the top crank pin 66 to the aligned upper and lower links 68 and70 and then to the upper end of the piston stem 32. If the pressureagainst the lower surface 38 of the inner piston 39 increases to a valuegreater than the compressive force applied to the upper end of thepiston stem 32 by the load springs 48, the second end of the pivot crank55 will be forced to rotate in a clockwise direction, as viewed in FIG.3, causing the upper end of the upper link 68 to be moved out ofalignment with the lower link pin 70. At this instant, the upper linkpin 72 will immediately move leftwardly, causing the upper and lowerlinks 68 and 70 to be moved out of vertical alignment, and therebyallowing the piston stem 32 to move upwardly. When the links 68 and 70are moved out of alignment, the upward movement of the piston 14 is, asdescribed above, impeded only by the flow of fluid from the enclosedchamber 28 through the apertures, or orifices, 41 into the enclosedchamber 33. Reset stem 73 may be used to facilitate manually moving thevalve into an open position when not under pressure or tripping thevalve when under pressure. As can be readily seen, line or working fluidis quickly expelled through exhaust port 42 facilitating the rapid flowof line fluid to the discharge conduit. The size and shape of exhaustport 42 may be of any a variety of configurations including an ellipse,oval, square or other opening shape. So long as the port is positionedso that no working fluid may pass when the valve is in the firstposition and upon activation is moved into a position to permit fluidflow, exhaust port 42 performs in the desired manner.

Because the upward movement of the piston 14 is hydraulically cushioned,there are no rebound, or “bounce” forces imposed on the piston 14, andaccordingly the piston will not undesirably reset itself. The valve 10embodying the present invention provides a second means for resettingthe valve, i.e., for engaging the first means for urging the piston 14into the first position. The second means includes a reset crank 76 thatis rotatably mounted in the bonnet 16, and has a handle 78 at one endextending externally of the bonnet assembly 16 and a roller 80 at asecond end disposed within the bonnet 16. The reset crank 76 is biasedby a reset lower spring 82 whereby the roller 80 at the second end ofreset crank 76 is maintained in a spaced, or clearance, position fromthe upper link 68, even when the upper link 68 is displaced fromvertical alignment with the lower link 70, i.e., when the piston 14 isat the second position.

To reset the valve 10, pressure must be relieved from the inlet port 22,whereupon the reset handle 78 can be manually rotated counterclockwiseto the rightward position shown in FIG. 2. This manual movement of thereset handle will cause the roller 80 to move the upper link 68 backinto vertical alignment with the lower link 70.

FIG. 4 shows an additional side view of the reset valve in accordancewith the present invention. Inner piston 39 is impeded in its downwardmovement by T-bar 93 composed of a horizontal member with an opening anda longitudinal bar affixed to said horizontal member. T-bar 93 ispositioned to prevent inner piston 39 from moving too far toward inletport 22 and potentially out conduit 18. T-bar 93 may be designed in anynumber of configurations that inhibit downward movement of inner piston39 to fit the circumstances present. In some applications, T-bar 93 maybe unnecessary altogether.

FIGS. 5A and 5B show piston 14 in plan and elevational views. FIG. 5Ashows two fluid flow apertures 41 previously described. Exhaust port 42is positioned on the side wall of piston head 30 to permit rapid fluidflow of discharge fluid upon activation of the valve. As previouslynoted, the size and location of exhaust port 42 may be varied dependingon the fluid mechanics of the subject fluid and in some cases may beomitted altogether depending on the forces present and the need forrapid evacuation of fluid.

FIGS. 6A, 6B, and 6C show crank 55. Load bearing bolt 58 goes throughsocket 86 and provides mechanical connection between crank 55 and theload bearing springs as previously described. Crank 55 has a cross bar88 that provides greater impact strength to the crank. At the back ofcrank 55, as seen in FIGS. 6B and 6C, there is tab or millstop 95positioned on each side of the crank. Link 70 shown in FIG. 3, forexample, is one of a pair of links that are positioned on opposite sidesof crank 55 and each engage their respective tabs or millstops on crank55 as described above. Tab or Millstop 95 of said crank 55 engages link70 at pin 72 connecting the upper and lower links shown in FIG. 2. Aspressure mounts on the crank caused by increasing line pressure on thepiston, the linkages engage the tab or millstop and at thepre-determined force provided by the piston against the load bearingsprings, the pin connecting the two linkages is nudged much like kickingsomeone in the back of the knee and the longitudinal arrangement of thetwo links is broken permitting the piston to move in an upwarddirection.

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the scope of the invention tothe particular form set forth, but on the contrary, it is intended tocover such alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims.

1. A reset relief valve comprising: a body having an inlet port, anoutlet port, and an interior wall partially defining an enclosedchamber; a piston disposed in said body and moveable between a firstposition at which said piston is disposed in fluid flow-blockingrelationship between said inlet port. and said outlet port of the bodyand a second position at which said piston is removed from said fluidflow-blocking position; said piston having a head which partiallydefines a first chamber and at least one flow passageway extendingbetween said chamber in said body and a second chamber having a lowersurface defined in part by a disc disposed in a cavity in said pistonhead; a bonnet attached to said body; a stem bushing disposed in saidbonnet in spaced relationship with the head of said piston, said stembushing having a surface defining another portion of said first chamberand an internal bore in which said stem is slidably supported; at leastone spring and at least two pivotally interconnected link membersoperatively disposed between said spring and said stem; and crankassembly for selectively engaging said pivotally interconnected linkmembers for urging said piston into said first position.
 2. A resetrelief valve as claimed in claim 1 wherein said crank assembly includesat least one spring and at least two pivotally interconnected linkmembers operatively disposed between said spring and said stem.
 3. Areset relief valve as claimed in claim 1 wherein said crank assemblyincludes a release shaft extending through a wall of said bonnet andhaving one end disposed in biased abutting contact with one of saidpivotally interconnected link members when said piston is at said firstposition.
 4. A reset relief valve as claimed in claim 1 wherein saidpiston has at least one cylindrical fluid communication path betweensaid area above said disc and said chamber above said piston.
 5. A resetrelief valve as claimed in claim 1 wherein said crank assembly has acrank composed of a first and second arm connected by a cross bar.
 6. Areset relieve valve as claimed in claim 1 wherein said first and secondchambers are filled with hydraulic fluid.
 7. A reset relief valve asclaimed in claim 6 wherein said fluid is oil.
 8. A reset relief valve asclaimed in claim 1 further comprising a longitudinal member disposed atthe bottom of said piston chamber to prevent downward movement of saiddisc.
 9. A reset relief valve as claimed in claim 8 wherein said memberis a T-shaped bar positioned along the longitudinal axis of the pistonat the inlet port.
 10. A reset relief valve comprising: a body having aninlet port, an outlet port, and an interior wall partially defining anenclosed chamber; a partially hollowed out piston disposed in said bodyand moveable between a first position at which said piston is disposedin fluid flow-blocking relationship between said inlet port and saidoutlet port of the body and a second position at which said piston isremoved from said fluid flow-blocking position; piston having a headwith opposed surfaces one of which defines a first chamber and at leastone fluid flow passageway extending through said head and providingfluid communication between said first chamber in said body and a secondchamber having a lower surface defined by an inner piston disposed insaid hollow portion of said piston head; a bonnet attached to said body;a stem bushing disposed in said bonnet in spaced relationship with thehead of said piston, said stem bushing having a surface defining anotherportion of said enclosed chamber and an internal bore in which said stemis slidably supported; and crank assembly for urging said piston intosaid first position.
 11. A reset relief valve as claimed in claim 9wherein said disc is comprised of a inner piston body attached to a cap.12. A reset relief valve as claimed in claim 9 further comprising alongitudinal member in said enclosed chamber for preventing downwardmovement of said disc.
 13. A reset relief valve as claimed in claim 9wherein said crank assembly has at least one tab for engaging saidpivotally interconnected link members at a link between said members.14. A reset relief valve as claimed in claim 9 wherein said piston headhas an opening through said hollow portion.
 15. A reset relief valve asclaimed in claim 9 wherein said crank assembly includes at least onespring and at least two pivotally interconnected upper and lower linkmembers operatively disposed between said spring and said stem whereinsaid lower link is substantially oval along its planar surface.
 16. Areset relief valve comprising: a body having an inlet port, an outletport, and an interior wall partially defining an enclosed chamber; apiston disposed in said body and moveable between a first position atwhich said piston is disposed in fluid flow-blocking relationshipbetween said inlet port and said outlet port of the body and a secondposition at which said piston is removed from said fluid flow-blockingposition; said piston having a partially hollow head and at least oneflow passageway extending between said chamber in said body and a secondchamber partially defined by a moveable disc in said piston head; abonnet attached to said body; a stem bushing disposed in said bonnet inspaced relationship with the head of said piston, said stem bushinghaving a surface defining another portion of said first chamber and aninternal bore in which said stem is slidably supported; at least onespring and at least two pivotally interconnected link membersoperatively disposed between said spring and said stem; and crankassembly for selectively engaging said pivotally interconnected linkmembers for urging said piston into said first position.
 17. A resetvalve as claimed in claim 16 further comprising an exhaust port in saidhollow portion of said piston head.